This invention relates generally to current-sense circuitry, and more particularly the invention relates to circuitry for sensing current through a high-voltage/high current switch.
Current-sensing is important for circuits operating at high-voltage or which have high currents to prevent overloading of the circuit. One such circuit is a switching DC-to-DC converter which uses current mode control. A decision to turn off the power switch can depend on the level of current flowing through the switch.
A circuit in accordance with the present invention allows fast and precise sensing of a current, even if a high-voltage is present when the switch is off or a spurious high current is present upon conduction of the switch.
Briefly, a circuit for sensing a load current comprises a first conductive path (e.g. switch) for passing a first portion of the current, a second conductive path for passing a second portion of a current and a scaled bias current, and an output circuit responsive to a generated voltage in the second conductive path and the bias current for producing a measure of the sensed current.
In a preferred embodiment the first conductive path comprises a load current switch and the second conductive path comprises a voltage divider including two serially connected field effect transistors having a common node. The output circuit includes a third field effect transistor with the generated voltage at the common node applied across the third field effect transistor. The second field effect transistor and the third field effect transistor have the same gate voltage applied thereto, thus the current through the two transistors are equal. Two transistors are provided in the output circuit including a first transistor for applying a current through the third field effect transistor and the second transistor providing an equal current as a first output. The current in the first transistor is a sample of the bias current and a scaled value of the sensed current. A current source can be provided for providing a scaled bias current as a second output, and means such as a differential amplifier receives a voltage derived from the first and second outputs and provides a measure of the sensed load current.
The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawing.